1. Field of the Invention
The present invention relates to an internal focusing zoom lens, more specifically to a high-power zoom lens in which the number of elements of the zoom lens is reduced while the structure thereof is simplified.
2. Description of the Related Art
In a zoom lens with a high zoom ratio (ratio of focal length variation), if the zoom lens is a front-lens-group advancing type zoom lens in which a focusing operation is performed by advancing the front (frontmost) lens group, the effective aperture of the front lens group becomes extremely large, which causes an increase in size of the mechanism for moving the front lens group. This causes an increase in size of the zoom lens and also an increase in mass of the elements of the AF mechanism which are to be driven, thus causing an increase in driving load. As a consequence, a problem arises in which the focusing speed reduces. In consideration of this drawback, a lens barrel with an internal lens group as a focusing lens group, i.e., a so-called internal focusing zoom lens has been proposed. In an internal focusing zoom lens, the aforementioned problem does not arise because the focusing lens group is not large in diameter; however, there is a problem with a focal point on the same object varying when the focal length varies by a zooming operation. In addition, each of the focal point at infinite distance and the focal point at the shortest distance varies by a variation in focal length of the zoom lens, and accordingly, there is a problem with the range of movement of the focusing lens group also varying when a focusing operation is performed. In order to overcome such a problem, conventional zoom lenses are provided with a focusing-lens-group advancing mechanism for originally changing the amount of movement of the focusing lens group in association with a zooming operation.
For instance, the zoom lens disclosed in Japanese Unexamined Patent Publication 2000-89086 (hereinafter referred to as Patent Document 1) is provided with a correction cam separately from a focus cam adopted to move a focusing lens group, and the correction cam is driven with the focus cam in association with a zooming operation to move the focusing lens group to an in-focus position.
Additionally, in Japanese Patent Document No. 3461224 (hereinafter referred to as Patent Document 2), the amount of movement of a focusing lens group is adjusted by moving a focus cam in an optical axis direction in association with a zooming operation to thereby change the usage area of the camming portion of the focus cam in response to variations of focal length, wherein the focus cam is for moving the focusing lens group when a focusing operation is performed.
In the technology disclosed in Patent Document 1, the correction cam is required in addition to the zoom cam and the focus cam, and accordingly, there is a problem with the number of elements of the zoom lens increasing. In particular, the correction cam needs to be positioned to be radially overlaid on the focus cam, which increases the diameter of the zoom lens. This increase becomes an obstacle to the design and production of a small-diameter zoom lens.
In the technology disclosed in Patent Document 2, it is advantageous for achieving a reduction in the number of elements of the zoom lens because no correction cam is required. However, in Patent Document 2, to drive a mechanism for moving the front lens group that is positioned radially outside of the focus cam, it is necessary to drive this moving mechanism by the focus cam that is moved by a zooming operation since the zoom cam is positioned radially inside of the focus cam. Therefore, to this end, the focus cam needs to be provided with the following two mechanisms: a mechanism for moving the focusing lens group via the zoom cam (e.g., a set of cam follower pins which engage with an associated set of cam grooves formed on the zoom cam), and a mechanism for moving the front lens group when the focus cam is moved by the zoom cam (e.g., a set of cam follower pins which engage with both a set of cam grooves of the focus cam and a set of cam grooves of a cam member for moving the front lens group). Therefore, according even to the technology disclosed in Patent Document 2, an increase in the number of elements of such two mechanisms cannot be avoided, which becomes an obstacle to the design and production of a small-diameter zoom lens just as in the case of Patent Document 1.